The present invention relates to gas turbine engines, and, more specifically, to a stator of a gas turbine engine.
In a gas turbine engine, air is pressurized in a compressor and mixed with fuel and ignited in a combustor for generating combustion gases having high temperatures. Energy is extracted from the combustion gases in stages of a turbine. The turbine powers the compressor and produces useful work, such as driving a generator to produce power, for example.
Since turbines are continuously exposed to the combustion gases during operation, cooling of turbine components is required. Bleeding a portion of the pressurized air from the compressor and channeling it through the turbine components often provides cooling air to accomplish cooling of turbine components. However, the cooling air is at a premium with respect to overall turbine performance, since useful work has already been done to the cooling air in the compressor. Therefore, it is desirable for turbine performance that an amount of air bled for nozzle cooling be kept to a minimum.
A typical gas turbine directly receives combustion gases from the combustor and includes an initial stage stator and a corresponding initial stage rotor having a plurality of rotor blades or airfoils extending radially outward from a supporting disk. Nozzles disposed around a circumference of each stator stage direct a flow of the combustion gases toward a row of corresponding rotor blades. After the combustion gases pass through the initial stage stator and the initial stage rotor, subsequent stage stators then direct the combustion gases through a corresponding row of rotor blades extending from corresponding subsequent stage rotors. The subsequent stage stators receive lower temperature combustion gases than the initial stage stator and therefore have different cooling requirements. Additionally, individual nozzles within each of the initial and subsequent stator stages often receive combustion gases at different temperatures.
The nozzles of the turbine are designed for durability with extensive lives measured in hours and/or cycles of operation. Such extended life is difficult to achieve since the nozzles are subject to various differential temperatures during operation, which create thermal stresses on the nozzles. Additionally, nozzles are subjected to oxidation or erosion, which are temperature driven, and coating spallation (when applicable), which is driven by both temperature and thermal stress. Suitable nozzle cooling is required to limit thermal stresses and peak metal temperatures to ensure a useful life. However, temperature distributions and heat transfer coefficients of the combustion gases channeled through each nozzle vary significantly and increase the difficulty of providing suitable nozzle cooling.
Ensuring that suitable nozzle cooling is provided to each nozzle is a difficult problem. Turbines often experience localized areas of high temperature within a particular stage. Circumferential and radial variations in combustion exit temperatures create the localized areas of high temperature. An area having a highest temperature relative to surrounding areas is referred to as a hot-streak. Location of a hot streak and the dynamics thereof are not easily predictable, thus applying sufficient cooling to areas in the hot streak is problematic and potentially expensive since complex cooling systems are often required. Rotor blades are typically not significantly impacted by the presence of a circumferential hot streak since their exposure to temperatures associated with the hot streak is limited by rotation of the rotor blades. However, nozzles of a particular stator stage may be exposed to hot streak conditions for extended periods and endure high temperatures and thermal stresses, which shorten nozzle life.
Since hot streak conditions must be considered, nozzle design engineers typically design all nozzles to be able to withstand worst-case temperatures associated with exposure to hot streak conditions. Additionally, maintenance practices have been developed to inspect and replace nozzles after a certain number of running hours, or to extract nozzles and swap their locations in an effort to equalize accumulated part life consumption among the nozzles. Designing a worst-case nozzle capable of extended exposure to hot streak conditions requires additional expense and/or cooling flow requirements. Furthermore, maintenance practices requiring routine replacement or relocation of nozzles add to both expense and system down time, and the need for additional cooling flow diminishes turbine performance.
Accordingly, it is desired to develop a method and system for reducing the impact of hot streak conditions on turbine design to decrease cooling requirements for turbines, which may in turn decrease nozzle manufacturing expense, reduce turbine down time due to nozzle inspection or replacement, and enhance turbine performance.